John C. Danforth was a Republican U.S. senator from Missouri from 1976 to 1995.

Many have said that President Trump isn’t a Republican. They are correct, but for a reason more fundamental than those usually given. Some focus on Trump’s differences from mainstream GOP policies, but the party is broad enough to embrace different views, and Trump agrees with most Republicans on many issues. Others point to the insults he regularly directs at party members and leaders, but Trump is not the first to promote self above party. The fundamental reason Trump isn’t a Republican is far bigger than words or policies. He stands in opposition to the founding principle of our party — that of a united country.

We are the party of Abraham Lincoln, and our founding principle is our commitment to holding the nation together. This brought us into being just before the Civil War. The first resolution of the platform at the party’s first national convention states in part that “the union of the States must and shall be preserved.” The issue then was whether we were one nation called the United States or an assortment of sovereign states, each free to go its own way. Lincoln believed that we were one nation, and he led us in a war to preserve the Union.

That founding principle of the party is also a founding principle of the United States. Even when we were a tiny fraction of our present size and breadth, the framers of our Constitution understood the need for holding ourselves together, whatever our differences. They created a constitutional structure and a Bill of Rights that would accommodate within one nation all manner of interests and opinions. Americans honor that principle in the national motto on the presidential seal: “e pluribus unum” — “out of many, one.” Today, the United States is far more diverse than when we were a nation of 3 million people , but the principle remains the same: We are of many different backgrounds, beliefs, races and creeds, and we are one.

The Republican Party has a long history of standing for a united country. Theodore Roosevelt raised up the ordinary people of his day and championed their cause against abusive trusts. Dwight Eisenhower used the army to integrate a Little Rock high school. George H.W. Bush signed the most important civil rights legislation in more than a quarter-century, a bill authored by Republican senators. George W. Bush stood before Congress and the nation and defended Muslims after 9/11. Our record hasn’t been perfect. When we have pushed the agenda of the Christian right, we have seemed to exclude people who don’t share our religious beliefs. We have seemed unfriendly to gay Americans. But our long history has been to uphold the dignity of all of God’s people and to build a country welcoming to all.

Now comes Trump, who is exactly what Republicans are not, who is exactly what we have opposed in our 160-year history. We are the party of the Union, and he is the most divisive president in our history. There hasn’t been a more divisive person in national politics since George Wallace.

It isn’t a matter of occasional asides, or indiscreet slips of the tongue uttered at unguarded moments. Trump is always eager to tell people that they don’t belong here, whether it’s Mexicans, Muslims, transgender people or another group. His message is, “You are not one of us,” the opposite of “e pluribus unum.” And when he has the opportunity to unite Americans, to inspire us, to call out the most hateful among us, the KKK and the neo-Nazis, he refuses.

To my fellow Republicans: We cannot allow Donald Trump to redefine the Republican Party. That is what he is doing, as long as we give the impression by our silence that his words are our words and his actions are our actions. We cannot allow that impression to go unchallenged.

As has been true since our beginning, we Republicans are the party of Lincoln, the party of the Union. We believe in our founding principle. We are proud of our illustrious history. We believe that we are an essential part of present-day American politics. Our country needs a responsibly conservative party. But our party has been corrupted by this hateful man, and it is now in peril.

In honor of our past and in belief in our future, for the sake of our party and our nation, we Republicans must disassociate ourselves from Trump by expressing our opposition to his divisive tactics and by clearly and strongly insisting that he does not represent what it means to be a Republican.

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After President Trump's most recent rhetoric about Charlottesville inflamed even more criticism, many Republicans stayed silent. But a handful of GOP lawmakers and now Trump's own economic adviser are directly criticizing him. (Jenny Starrs/The Washington Post)